NEW DELHI (Reuters) - Revelations in a new book by former Congress Party stalwart K. Natwar Singh have put Sonia Gandhi on the defense this week, in the latest challenge to her leadership of the embattled party and India's foremost political dynasty.

In the autobiography "One Life is Not Enough," released on Friday, Singh paints an unflattering picture of the 67-year old Congress party head, labelling her an at-times "Machiavellian" leader who has been "coarsened" by politics.

The remarks come at a fraught time for Gandhi, having recently presided over her party's slide from a decade in power to its worst-ever defeat in national elections in April and May.

Singh, a former external affairs minister who was expelled from the party, places the blame for the drubbing on Gandhi, alleging her grip on the Congress Party has been both complete and destructive.

"What Sonia Gandhi has achieved is to reduce the Congress, one of the greatest political parties of the world, to a rump of forty-four members in the Lok Sabha," he wrote.

Unsurprisingly, Congress has come out swinging in defense of its leader.

"We note the necessity to sensationalize such matters on the eve of publication because that is vital to increase sales," Congress spokesman Abhishek Singhvi said in a July 31 press briefing.

"It is, of course, highly regrettable that persons who have enjoyed high positions of power and entrusted to them by and on behalf of the Congress party ... misuse and distort such confidentiality for commercial purposes."

The passage in Singh's book that has created the biggest stir is one in which he reports Gandhi declined the prime ministership after leading Congress to victory in 2004 because her son, Rahul, objected. Rahul Gandhi is now vice president of the party.

"Rahul was vehemently opposed to his mother becoming Prime Minister, fearing that she would lose her life, much like his grandmother and his father," Singh wrote.

"He had his way. That was the reason for her not becoming Prime Minister."

After the 1984 assassination of Prime Minister Indira Gandhi, her son Rajiv Gandhi, Sonia's husband, took over as prime minister. He was assassinated by a suicide bomber in 1991.

On Thursday, in an unusual public comment, Sonia Gandhi struck back at Singh's claims, telling reporters that she would write a book to give her own account of events.

"The only way the truth will come out is if I write," she said, according to NDTV.

Analysts have said the big win by the Bharatiya Janata Party (BJP), which secured 282 seats in India's lower house of parliament compared with Congress's 44, had as much to do with the well-run campaign of Prime Minister Narendra Modi as the lacklustre re-election bid by Congress.

After Congress's defeat, both Sonia and Rahul Gandhi offered to step down as leaders of the party, but the offers was rejected by party bosses.